US Secretary of State John Kerry on Monday offered his condolences to Serbian Prime Minister Aleksandar Vucic over the two kidnapped Serbian diplomats believed to have been killed in a US air strike in Libya, Belgrade said.

Last week the US targeted an Islamic State (IS) training camp near the Libyan coastal city of Sabratha, killing dozens of people including an IS operative the US said was behind the mass murder of tourists on a Tunisian beach.

However, Belgrade said the strike's victims also included two officials from Serbia's embassy in Libya, Sladjana Stankovic and Jovica Stepic, who had been taken hostage in the area in November.

The Pentagon said Saturday it had "no information" indicating that the attack had led to the deaths of two Serbians and that the circumstances of their deaths "remained unclear".

In a telephone conversation on Monday Kerry "expressed condolences to Vucic and the families over the death of Sladjana Stankovic and Jovica Stepic," the Serbian government said in a statement.

Kerry "told the prime minister that he would inform the Serbian government ... about all the details of an investigation conducted by the US and its services, about the murder of Serbian diplomats," it said.

Vucic on the weekend said that the pair "would have been released, had they not been killed".

US officials said the raid likely killed Noureddine Chouchane, also known as "Sabir," who along with other militants had been planning attacks against American and other Western interests.

Chouchane is suspected of being behind both the beach attack in July 2015 near the Tunisian city of Sousse that killed 38 tourists, including 30 Britons, and an attack on the National Bardo Museum in Tunis that killed 21 tourists and a policeman in March 2015.

Both attacks were claimed by the IS group.

Sabratha, which lies 70 kilometres (42 miles) west of Tripoli, is considered a stronghold of militancy in Libya, where fighters are trained in camps before launching deadly attacks in other countries.

The country spiralled into chaos after longtime dictator Moammar Gaddafi was overthrown and killed in October 2011 with NATO support, with two governments now vying for power and armed groups battling to control vast energy resources.

Belgrade maintains an embassy in Tripoli, and Serbian citizens - mostly doctors, other medical staff and construction workers - have been working in Libya for decades due to close bilateral relations during Gaddafi's rule.